This invention relates to valves and more particularly to through conduit gate valves of the rising stem type and still more particularly to a backseat assembly for sealing the stem and bonnet of an expanding gate valve.
A through conduit gate valve of the rising stem type is well known in the art and has been specially designed to control the flow in conduits carrying fluid such as oil or gas which may be under high pressure. A through conduit gate valve is one which, with the gate in the open position, leaves the conduit unobstructed to flow.
One type of sliding gate valve is an expanding gate valve because it includes a ported gate mechanism which has double-tapered gate elements, namely, a gate and a segment which reciprocate between opposed valve seats around the through conduit. The two double-tapered sections have opposed, contacting inner faces which diverge outwardly from a central portion to provide two sets of wedging surfaces. Only the gate is attached to stem with the segment attached to the gate by linkage. Because of this construction, when the segment is at the end of its opening or closing movement, the gate is shifted relatively from a center position, along the line of travel of the gate mechanism, to effect a wedging action between the gate and segment to expand the gate mechanism and force the outer sealing faces of the gate and segment against the valve seats.
The expanding gate valve differs from a standard slab gate or a wedge gate valve in that the amount of travel of the gate is determined by the expansion of the gate and segment against the valve seats due to the wedging action. During actuation, the gate and segment move together until the segment engages the valve bonnet or housing thereby preventing its further movement. The stem of the valve continues moving causing the gate to continue its travel and expand outward into a sealing position with the valve seats. The gate mechanism operates the same in both the open and closed positions.
A packing is provided around the stem for providing a dynamic seal between the stem and the bonnet or housing of the valve. Industry has shown concern that the dynamic seal provided by the packing will allow fugitive emissions into the atmosphere. The Environmental Protection Agency is monitoring fugitive emissions of valves being used in industrial plants. Depending upon the type of fluid passing through the valve, the dynamic seal provided by the packing allows some minute leakage around the stem. Such minute emissions are inherent whenever a dynamic seal is required for sealing a moving surface such as the movement of the stem in and out of a gate valve which acts as a pressure vessel.
In the case of a fire, one of the first seals to be destroyed is that of the packing around the stem. This is particularly true in a smaller valve such as a 4" valve. Once the packing is destroyed, if there is no backseat ring, the hydrocarbon products passing through the valve then feed the fire by leaking around the stem and bonnet. The metal-to-metal seal of a backseat ring will then provide a seal if the packing is destroyed by fire.
Prior art expanding gate valves do not provide a redundant stem seal in the operating position of the valve, typically the open position. See for example U.S. Pat. Nos. 2,002,780; 2,148,628; and 2,479,124. A redundant stem seal assists in preventing fugitive emissions into the atmosphere and provides a backup seal in case of a fire.
Backseats on the stem are used in slab type gate valves such as shown in U.S. Pat. Nos. 3,013,769 and 3,412,750 and in non-standard split gate valves such as shown in U.S. Pat. Nos. 2,502,689 and 3,026,897. Typically, the stem includes an integral backseat shoulder which serves both as a backseat ring and a travel stop.
A backseat ring cannot be fixed to the stem of a through conduit gate valve of the rising stem type having expanding gates. An integral backseat disposed on the stem of an expanding gate valve would prevent the continued upward travel of the gate since the backseat ring would limit the upward travel of the stem when the backseat ring engaged the seat on the bonnet. The backseat ring cannot interfere with the travel of the gate.
The present invention overcomes the deficiencies of the prior art.